The present invention relates to an oceanographic sensor system and technique and more particularly to a sensor mounting system and technique wherein the sensors, such as transducers, are indirectly attached to a vessel and independently supported by floats.
Hydrographic surveys have traditionally been conducted by running a survey boat in parallel lines across the area to be surveyed. Since World War II, survey vessels have incorporated sonar (sound navigation and ranging) type systems, which measure depth by employing transducers which emit and receive sounds reflected from the bottom. Although the use of a transducer equipped system is clearly beneficial, problems remain. For example, as the survey vessel travels in parallel lines and the distance between each line is a function of the number of passes a ship is able to make in a given area, various bottom features, such as isolated pinnacles, wrecks and other obstructions, if not entire areas, remain uncharted, absent an excessive number of passes by the survey vessel. Consequently, there is an increasing interest in the use of "swath" sounding systems in which an array of transducers are deployed perpendicular to the direction of the vessel's travel such that a "swath" is surveyed at each pass of the vessel. Unfortunately, previously developed swath sounding systems have suffered from one or more defects which prevent their wide acceptance.
Previously, the transducers were normally mounted on booms ranging in length from 30 to 50 feet and hinged to the side of the ship, so that they could be swung back when not in use. As the array of transducers was attached to the vessel, the distance of each transducer from the bottom varied with the movement of the vessel, thus aggravating the already difficult task of determining actual depth relative to position on the bottom. For example, a roll of only three degrees in the hull can raise a transducer mounted on a 40 foot boom as much as two feet or more, thus creating an error of the same magnitude. The importance of correcting such an error is obvious, when one considers that supertankers and other ships often operate with an underkeel clearance of less than one meter, even though overall draft may approach 30 meters.
Some have suggested the addition of a roll sensor to correct this problem. However, this has been generally rejected since it not only fails to eliminate the problem, but adds substantially to the cost and complexity of the system. Others have mounted the booms below the waterline in an attempt to reduce the amount of roll, apparently employing the booms as stabilizers. However, this likewise fails to eliminate the problem and also adds substantially to cost, since it increases drag on the vessel and requires strengthened joints and a steel hull of some size in which proper mounts can be permanently installed.
Previous swath sounding systems have also often lacked in ease of installation, often requiring extensive structural modification to the vessel to properly secure the transducer array. This in turn not only limited the type of vessel available for surveying, but also added to the expense of surveying. Additionally, those vessels which pushed a supported boom in front of the vessel in order to provide ease in installation have employed rigid direct support from the vessel, thus transmitting the full effect of the vessel roll to the sensor array. Moreover, the width of the swath is still limited by several factors including the size of the vessel and the strength of the direct supports, as is the case with all previous swath sounding systems.
Finally, previous swath systems have failed to employ other sensors. This is possibly due either to the sensitivity of such devices to the roll and pitch of the vessel or to the effects of the structural materials contained in the hull and swath system supports, as in the case with magnetometers, which interact with the metal in the hull and swath system supports.